Win32: Add GLFW_WIN32_KEYBOARD_MENU

This platform specific window hint enables access to the Windows window
menu via the keyboard shortcuts.

src/win32_window.c

@@ -699,7 +699,12 @@ static LRESULT CALLBACK windowProc(HWND hWnd, UINT uMsg,
 
                 // User trying to access application menu using ALT?
                 case SC_KEYMENU:
-                    return 0;
+                {
+                    if (!window->win32.keymenu)
+                        return 0;
+
+                    break;
+                }
             }
             break;
         }
@@ -731,6 +736,10 @@ static LRESULT CALLBACK windowProc(HWND hWnd, UINT uMsg,
             }
 
             _glfwInputChar(window, (unsigned int) wParam, getKeyMods(), plain);
+
+            if (uMsg == WM_SYSCHAR && window->win32.keymenu)
+                break;
+
             return 0;
         }
 
@@ -1275,6 +1284,7 @@ static int createNativeWindow(_GLFWwindow* window,
     }
 
     window->win32.scaleToMonitor = wndconfig->scaleToMonitor;
+    window->win32.keymenu = wndconfig->win32.keymenu;
 
     // Adjust window rect to account for DPI scaling of the window frame and
     // (if enabled) DPI scaling of the content area